A few zombies are in the starting room, so kill them and grab the berserk pack in the corner. Open the door and go around, ignoring the blue door for now. The next room has a few enemies, as well as the blue key. There is a super shotgun hiding behind one of the crates here. Return to the blue door and open it, disposing of the enemies inside. There is a lift you can use to head up. At the top, follow the path killing enemies until you reach the exit switch.